Foam has been used in a variety of different coating and adhesive applications. For example, Scholl et al U.S. Pat. No. 4,059,714 discloses applying a thermoplastic adhesive having a gas dissolved in the thermoplastic adhesive. When dispensed at atmospheric pressure the gas is released from solution and a foam is delivered to a substrate for bonding with another substrate. The substrates are bonded by pressing the foamed adhesive therebetween to destroy the cellular structure and distribute the adhesive between the two substrates.
Foaming has also been used as a means to reduce the amount of solvents required to atomize coating compositions. Coating compositions contain a substantial amount of solids which usually are dissolved in a solvent in order to provide for better dispersion of the solids for application to a substrate. Without adequate solubilization and atomization, the formed coating would have a blotchy appearance. Solvents have been used to dissolve the solids thereby making them more fluid, less viscous and easier to atomize. Cobbs, Jr. et al U.S. Pat. No. 4,247,581 discloses foaming coating compositions in order to reduce the need for solvents. According to Cobbs, a stream of foamed coating composition is first formed and then an external atomizing force breaks up the foam on its way to a substrate. Other patents have issued on foaming methods and apparatus. A special foam generating nozzle is disclosed in Rehman et al U.S. Pat. No. 4,553,701 and U.S. Pat. No. 4,630,774 discloses a nozzle designed to air atomize foam emitted from the nozzle. Cobbs et al U.S. Pat. No. 4,505,957 likewise discloses foam atomization upon emission from the nozzle. Cobbs U.S. Pat. No. 4,423,161 and related U.S. Pat. No. 4,301,119 disclose foam coating. Cobbs U.S. Pat. No. 4,608,398 relates to foam application of thermosetting compositions and Cobbs U.S. Pat. No. 4,527,712 discloses an apparatus for dispensing foamable compositions.
The prior methods of atomizing coating compositions have used foaming as a means to an end, i.e., where the foam is destroyed after delivery from a spray nozzle before it hits the substrate, or immediately thereafter. Particularly in the above Cobbs patents directed to atomization, the objective was to form a smooth continuous coating composition without any surface irregularities such as pin holes on a substrate. For this reason the foam must be destroyed while traveling from the atomizer to the substrate or upon deposition and film forming.